1. Field of the Invention
The present invention relates to a light amount-adjusting member in a light amount-adjusting device, for example, a light-reducing filter, used in a photographing apparatus, such as a camera, and various other optical instruments. Particularly, the present invention relates to a production process of a light amount-adjusting member suitable for use in production of a neutral density filter in the visible light range (400 to 700 nm), a light amount-adjusting member obtained by the process, and a light amount-adjusting device and a photographing apparatus using the light amount-adjusting member produced by the process.
2. Related Background Art
Conventionally, various optical instruments, such as digital and video cameras, use a diaphragm device in order to adjust the amount of incident light. In particular, in such devices, the amount of incident light is adjusted using diaphragm blades. When the diaphragm aperture becomes very small against a highly luminous subject, however, resolution deteriorates because of diffraction. Thus, the amount of passing light is adjusted by using a neutral density filter (hereinafter abbreviated as “ND filter”) or the like as a light amount-adjusting member in addition to the diaphragm, thereby preventing the image quality from deteriorating. More specifically, the diaphragm is constructed that the ND filter, which is a member separated from the diaphragm blades, is fitted to a part of the diaphragm blade with an adhesive, whereby a diaphragm aperture is maintained at a certain size without excessively being stopped down. The ND filter is located on an optical axis to limit the amount of passing light. Further, when an ND filter having a gradient in its light amount-adjusting function (hereinafter referred to as “density gradient”) is used, and this filter is moved on the optical axis, further light amount adjustments may be conducted. Alternatively, there have been proposed various diaphragm devices in which an ND filter is not adhered to a diaphragm blade, but acts independently.
The ND filter, a light amount-adjusting member in a light amount-adjusting device as described above, may, for example, have a metal film or a dielectric film formed thereon by vapor deposition etc., or have a multi-layered metal or dielectric films formed by lamination, because these materials have good optical characteristics and excellent durability. Other production processes include those of a type where a light-absorbing dye or pigment is mixed and incorporated into glass, cellulose acetate, PET or the like, which is a transparent film-forming material, and of a type in which a light-absorbing dye or pigment is applied to a transparent base material composed of the above-described material. Further, as disclosed in Japanese Patent Application Laid-Open No. H5-173004, a production process of an ND filter using a silver halide film has also been proposed. In addition, a light amount-adjusting member having a density gradient is utilized as a light amount-adjusting device for a laser beam printer or the like as disclosed in, for example, Japanese Patent Application Laid-Open No. H11-14923, although its spectral transmission characteristics are not constant. In Japanese Patent Application Laid-Open No. 2002-268120, it is described that a light interrupting portion of an optical functional filter is formed by printing with a black coating or the like, and the printing system includes various systems, such as a transfer system, an ink-jet system, a stamping system and a system that applies a photosensitive coating to a surface of a plate to expose it to light. It is also described that the optical functional filter functions as a near infrared-absorbing filter, an optical low-pass filter, an ND filter and a color tone filter.
However, the ND filter obtained by forming the coating by vapor deposition has a problem in that its production cost becomes high because the production apparatus becomes large in scale, and the production process is complicated. The process that can be used to form the ND filters by incorporating a dye or pigment into the material during film formation or the process that forms ND filters by applying a dye or pigment to the surface of the base material can produce a filter having a uniform density. However, it is very difficult to produce a filter having a density gradient. According to the production process using the silver halide film, a filter having a density gradient can be produced. However, the production process involves a problem in that light scattering is caused by silver particles remaining in the film to deteriorate the optical characteristics. In the case where the ND filter is produced by the ink-jet system, it has been difficult to produce an ND filter having almost constant spectral characteristics in a visible range of from 400 nm to 700 nm, particularly from 600 nm to 700 nm, while retaining ejection stability in ink jet recording.